expand

Override this method to specify how this PTransform should be expanded
on the given InputT.

NOTE: This method should not be called directly. Instead apply the
PTransform should be applied to the InputT using the apply
method.

Composite transforms, which are defined in terms of other transforms,
should return the output of one of the composed transforms. Non-composite
transforms, which do not apply any transforms internally, should return
a new unbound output and register evaluators (via backend-specific
registration methods).